A power supply apparatus for use with an arc utilizing device may include an inverter, for reduction of the size and weight of the apparatus. One of such power supply apparatuses includes a pair of inverters. An input AC voltage applied to the power supply apparatus is rectified by an input-side rectifier and smoothed by a capacitor into a DC voltage. The DC voltage is applied to the inverters, and converted into high frequency voltages. The high frequency voltages are voltage-transformed by a transformer and rectified by an output-side rectifier into a DC voltage for application to a load. By converting DC voltages into high frequency voltages, a small transformer can be used, whereby the power supply apparatus can be downsized as a whole.
When an input AC voltage to be applied to such power supply apparatus is high, e.g. 400V, the output voltage from the input-side rectifier, i.e. the input voltage to the inverters, is high (at .sqroot.2*400.apprxeq.565 V). Thus, the inverters require control elements, for example, switching elements such as IGBTs, MOSFETs and bipolar transistors, which have a breakdown voltage of 1200V or so. Switching elements having a breakdown voltage of e.g. 1200V, however, are less commercially available than, and twice or more as expensive as control elements having a breakdown voltage of e.g. 600V. Furthermore, switching elements having a breakdown voltage of 600V can be used at a higher frequency than elements having a breakdown voltage of 1200V. Therefore, use of switching elements having a breakdown voltage of 600V or so can realize a more compact power supply apparatus.
U.S. Pat. No. 5,272,313 issued on Dec. 21, 1993, assigned to the same assignee as the present application, discloses a power supply apparatus for use with an arc utilizing device in which two inverters using switching elements having a breakdown voltage of 600V or so are coupled in series. Such power supply apparatus can operate from a high input voltage.
FIG. 1 shows a portion of the power supply apparatus disclosed in the aforementioned U.S. patent which is related to the present invention. In FIG. 1, a three-phase high commercial AC voltage of e.g. 400V is applied between input terminals 1a, 1b and 1c and rectified by an input-side rectifier 2. A pair of smoothing capacitors 3a and 3b are coupled in series between output terminals 2a and 2b of the rectifier 2. The voltage rectified by the input-side rectifier 2 is smoothed by the smoothing capacitors 3a and 3b, and converted into a DC voltage.
A DC voltage across the smoothing capacitor 3a is applied to an inverter, e.g. a half-bridge type inverter 4a. The inverter 4a has switching elements (e.g. IGBTs) 7a and 8a coupled in series. A series combination of capacitors 5a and 6a is coupled in parallel with the series-coupled switching elements 7a and 8a. A flywheel diode 9a is coupled in parallel with the emitter-collector conduction path of the switching element 7a, and a flywheel diode 10a is coupled in parallel with the emitter-collector conduction path of the switching element 8a.
Similarly, a DC voltage developed across the other smoothing capacitor 3b is applied to an inverter 4b. The inverter 4b also has switching elements (e.g. IGBTs) 7b and 8b coupled in series. A series combination of capacitors 5b and 6b is coupled in parallel with the series-coupled switching elements 7b and 8b. A flywheel diode 9b is coupled in parallel with the emitter-collector conduction path of the switching element 7b, and a flywheel diode 10b is coupled in parallel with the emitter-collector conduction path of the switching element 8b.
The switching elements 7a and 8a of the inverter 4a are alternately turned on and turned off (i.e. made conductive and non-conductive) in response to control signals, e.g. pulse drive signals, applied thereto from a switching element driving unit 31, to convert the received DC voltage into a high frequency voltage. The switching elements 7b and 8b of the inverter 4b operate in the same manner as the elements 7a and 8a.
The high frequency voltages from the inverters 4a and 4b are applied respectively to a primary winding 11P of a transformer 11 and a primary winding 12P of a transformer 12. The voltage-transformed high frequency voltages are induced in a secondary winding 11S1 of the transformer 11 and a secondary winding 12S2 of the transformer 12, respectively. The induced high frequency voltages are rectified by an output-side rectifier comprising rectifying diodes 13a, 13b, 14a and 14b, smoothed by a smoothing reactor 15, and applied through output terminals 16P and 16N to a load, e.g. a workpiece and a torch.
An output current detector 32 detects an output current supplied to the load. The output current detector 32 provides a signal representative of the detected output current to an error amplifier 51. The error amplifier 51 receives also a signal representative of a desired output current set through an output current setting unit 33. The error amplifier 51 determines the difference between the desired-output-current representative signal and the detected-output-current representative signal, and provides an error signal representative of the difference to the switching element driving unit 31. The switching element driving unit 31 determines a conduction period during which each of the switching elements 7a, 8a, 7b and 8b is turned on, such that the error signal is zero. The sum of the conduction period and a non-conduction period of each switching element is a predetermined constant value. In other words, the switching elements 7a, 8a, 7b and 8b are PWM controlled. The transformer 11 has a secondary winding 11S2 in addition to the secondary winding 11S1. A high frequency voltage induced in the secondary winding 11S2 is rectified by a full-wave rectifier 18 including diodes 18a-18d, and applied through a resistor 20 to the input of the inverter 4b.
Similarly, the transformer 12 has a secondary winding 12S2 in addition to the secondary winding 12S1. A high frequency voltage induced in the secondary winding 12S2 is rectified by a full-wave rectifier 17 including diodes 17a-17d, and applied through a resistor 19 to the input of the inverter 4a.
Thus, the output voltage of the inverter 4a is fed back through the secondary winding llS2 of the transformer 11 to the input of the inverter 4b, and the output voltage of the inverter 4b is fed back through the secondary winding 12S2 of the transformer 12 to the input of the inverter 4a.
Such feedback of the output voltages of the inverters 4a and 4b can keep the voltages of the capacitors 3a and 3b equal regardless of the difference in capacitance or leak current between the capacitors 3a and 3b. Let it be assumed, for example, that the input voltage to the inverter 4a is higher than the one to the inverter 4b. Then, the voltage applied across the primary winding 11P of the transformer 11 is higher than the one across the primary winding 12P of the transformer 12. Therefore, the voltage induced in the secondary winding 11S2 of the transformer 11 becomes higher than the voltage induced in the secondary winding 12S2 of the transformer 12. The higher voltage induced across the secondary winding 11S2 is applied through the resistor 20 to the primary winding 12P of the transformer 12 supplied with the lower input voltage, whereas the lower voltage of the secondary winding 12S2 is applied to the primary winding 11P of the transformer 11 supplied with the higher input voltage. This can make equal the voltages applied across the primary winding 11P and the primary winding 12P.
In the power supply apparatus as stated above, it is desirable to promptly cancel the above-stated input voltage imbalance and make the voltages balanced. For that purpose, the resistors 19 and 20 should have a low resistance. The resistors 19 and 20 conduct a large current and should not be damaged by such large current. Such resistors are relatively large in size. Use of the large-sized resistors increases the size of the power supply apparatus, and offsets the advantage provided by the use of the inverters that can downsize the power supply apparatus.
An object of the present invention is to provide a power supply apparatus for use with an arc utilizing device which can provide balanced input voltages to inverters used therein and can still be compact in size.